java:强相关的类对

Question

我有一对彼此相关的类:

Msg1 <-> Data1
Msg2 <-> Data2
...

其中所有“DataX”都扩展了父类(super class)“Data”，而“MsgX”则扩展了类“Msg”。这些父类(super class)目前(简化):

abstract class Data< MSG extends Msg<? extends Data<MSG>>> {
       MSG msg;
       void modify( MSG msg ) {
           this.msg = msg;
       };   
}

和:

abstract class Msg< DATA extends Data<? extends Msg<DATA>>> {
    abstract DATA createData();

    void modifyData( DATA data ) {
        data.modify( this );
    };  
}

但是，Data 和 Msg 这两个类的编译失败，并在语句“data.modify( this );”处出现以下错误:

Msg.java:5: error: method modify in class Data<MSG> cannot be applied to given types;
            data.modify( this );
                ^
  required: CAP#1
  found: Msg<DATA>
  reason: argument mismatch; Msg<DATA> cannot be converted to CAP#1
  where DATA,MSG are type-variables:
    DATA extends Data<? extends Msg<DATA>> declared in class Msg
    MSG extends Msg<? extends Data<MSG>> declared in class Data
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Msg<DATA> from capture of ? extends Msg<DATA>
1 error

有什么建议吗？我已经尝试了数十种变体，添加和删除“？”、“扩展”……但它们都以某种方式失败了。

Answer 1

Java有时就是这样🤷。基本上，您需要做的是定义一个具体的类型参数，该参数引用所声明的类，而不是使用通配符。我的意思是，而不是

abstract class Data<M extends Msg<? extends Data<M>>> {}
abstract class Msg<D extends Data<? extends Msg<D>>> {}

将其声明为

abstract class Data<D extends Data<D, M>, M extends Msg<M, D>>
abstract class Msg<M extends Msg<M, D>, D extends Data<D, M>>

准确地说，像这样定义基类

abstract class Data<D extends Data<D, M>, M extends Msg<M, D>> {
       M msg;
       void modify( M msg ) {
           this.msg = msg;
       };
}

abstract class Msg<M extends Msg<M, D>, D extends Data<D, M>> {
    abstract D createData();

    void modifyData( D data ) {
        data.modify( (M) this ); // <- you still need to cast `this` though
    };  
}

然后你可以像这样声明子类:

class FooData extends Data<FooData, FooMsg> {
}

class FooMsg extends Msg<FooMsg, FooData> {
    @Override FooData createData() {
        return new FooData();
    }
}